The work proposed involves study of the oxygen-requiring steroid converting enzymes to be studied primarily in rat testis microsomes. The two enzymes to be studied are the 17,20-lyase and the 17 alpha- hydroxylase. Initial studies have determined that the lyase is a P-450 enzyme which employs cytochrome c reductase as an electron carrier. Furthermore, the two enzymes appear to employ different species of P- 450. Work in progress includes an attempt to resolve and reconstitute these microsomal enzymes. A survey of substituent groups of steroids that affect binding of these molecules to microsomal P-450 is also in progress.